Filter Shape Picture ( Re:BITX20A board finished (report JL1KRA)


Hi, All again,

Thanks to previous filter discussion around November 7th,2007.
Back to basic, I red W7ZOI article in W1FB's Design Notebook(ARRL).
As recommendation, I replaced C54,55,56 to philips trimmer
as in picture,

I reached C54=82pF, C55=6pF, C56=82pF and changed it to
fixed capacitor. fc=11001000Hz

The ripple in bandpass was minimized less than 3dB.
After this, setting carrier OSC to 109995000Hz,
2-tone injection caribration give me a good result.
Now the both tone are modurated same level.
Still there is some instability the MOS-FETs behaviour
in higher drive level. It's better than before.

The above capacitor value are my case, I am not sure
this will work in other case.
For those who will adjust C54,55,56, I recommend you to
solder these in bootom side of the board. Once these are
soldered between the tall crystals, they are difficult to
pull out.

JL1KRA Junichi Nakajima

--- In BITX20@..., "qrpjl1kra" <mx6s@...> wrote:

Hi, Robby and AH6GI

Thanks your comments,
My chasis preparation need more time. So, here is some bench
pictutes in progress.

In this weekend I am doing filter alignment.
Since I couldn't adjust TX carrier point correctly
with my 2 tone generator, I used Frequency Responce Mesurement System
(FRMS) and checked the BTX20A filter.
The FRMS is popular homebrew tool among JA QRPer using DDS and AD8307.
Without spectrum analyzer this is a good tool to see filer shape.

As you see the first pic, the bandpass has 10dB ripple.
DF5SF fileter pics shows similar shape. I suppose the
BTX20As are showing same character.

DF5SF changed C54,55,56 to 150,200,150pF from original 82,100,82pF.
I am now reaching 68,0,68pF to minimize the ripple. But
the skirt of the filter is worser than before to supress carrier.

May be this problem had been discussed in this list.
I am just arrived the point. I will check the previous messages

Junihi Nakajima JL1KRA

--- In BITX20@..., "ah6gi" <ah6gi@> wrote:

--- In BITX20@..., "R R Robson" <rrrobson@> wrote:

Great, Junichi! Hope to see your pix soon.
Yes, please take pictures!

I too have to finish other work, an SB-303 with a bad LMO. This is an
easy fix but it takes time to get the LMO out, replaced, and then
align the drive mechanism.

de ah6gi/4

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